1. Technical Field
The present invention relates to a holographic recording medium and recording method for recording a hologram, in particular to a data-rewritable hologram recording medium and recording method.
2. Description of the Related Art
Recently holographic memories which take advantage of the high density recordability of holograms are being actively investigated for use as the next generation of high-speed, high-capacity memory. Hitherto, inorganic photorefractive crystals, typified by lithium niobate (LiNbO3), have been used for recording materials in data-rewritable hologram recording media. However, refractive index changes of the inorganic photorefractive crystal are as small as 1 in 10,000, and the recording medium is forced to be as thick as 1 cm. Accordingly, it is pointed out that the material is liable to cause optical aberration and handling of the material is difficult.
In relation to the problems above, a holographic memory taking advantage of large changes in refractive index of organic polymer materials has recently attracted interest. For example, a holographic recording medium proposed in Japanese Patent Application Laid-Open (JP-A) No. 10-340479 comprises a polymer film containing a photoisomerizable molecule such as azobenzene and the like, wherein the polymer film is selectively photoisomerized by irradiation with polarized light to the photomerizable molecule, and a hologram is recorded by taking advantage of the high optical anisotropy (birefringence and dichroism) induced in the polymer film by the photoisomerization.
Azobenzene exhibits trans-cis photoisomerization under irradiation with light. The trans isomer and cis isomer are shown by the chemical formulae (a) and (b), respectively shown below:

While azobenzene itself exhibits optical anisotropy, as shown in FIG. 7A, a polymer film, prepared by applying a polymer solution in which azobenzene is dispersed, is isotropic as a whole as a result of the isotropic conformation of the solution.
When a linearly polarized light having a wavelength sensitive to azobenzene is irradiated as a pumping light, as shown in FIG. 7B, due to dichroism of the azobenzene molecules, only azobenzene molecules oriented in the same direction as the polarization direction of the pumping light (polarization axis) absorb the light and are isomerized into cis isomers. Since the cis isomer is thermally unstable, it is isomerized back again into the trans isomer by a thermal reversion reaction. The resulting trans isomers may be oriented in either the same direction as, or perpendicular to, the polarization axis. However any trans isomers oriented in the same direction as the polarization axis are isomerized once again into the cis isomer, and then isomerized back into the trans isomer thereafter by the thermal reversion reaction.
The azobenzene molecules finally become to be oriented in a direction where the molecules have a smaller light absorption ratio to the pumping light (perpendicular to the isomerization axis) as shown in FIG. 7C, by repeating trans-cis-trans isomerization cycles. Orientation of the polymer is also changed by the isomerization and orientation changes of the azobenzene molecules, whereby macro optical anisotropy is induced in the polymer film.
However, in the holographic recording medium taking advantage of the photoisomerization reaction as described above, optical anisotropy induced in the polymer film is not stabilized (i.e., a fixing process is not carried out) after recording a hologram, so that the recording medium remains rewritable. Accordingly, there arises a problem in that optical anisotropy is induced by exposing the recording medium to external light, such as the sunlight, after recording, and the recorded hologram can be destroyed.